Leaky SAW resonator and method

Abstract

A SAW ladder filter includes grating pads extending from opposing bus bars and interdigital transducer electrodes extending from the grating pads for defining an acoustic aperture. The metalization ratio for the grating pads is greater than that for the interdigital transducer electrodes. Reflector electrodes are disposed on opposing longitudinal sides of the interdigital transducer electrodes. Operation of the filter results in a velocity of the SAW along the grating pads slower than a SAW velocity along the plurality of interdigital transducer electrodes, thus producing a wave guiding effect for optimizing SAW propagation within the acoustic aperture. The grating pads may be longitudinally offset from cooperating interdigital transducer electrodes using minor bus bars.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application claims the benefit of U.S. Provisional Application No. 60 / 663,828 for “Leaky SAW Resonator and Method” having filing date Mar. 21, 2005, the disclosure of which is incorporated herein by reference in its entirety, all being commonly owned.FIELD OF INVENTION[0002]The present invention generally relates to surface acoustic wave devices, and particularly to surface acoustic wave resonators having improved energy characteristics.BACKGROUND OF THE INVENTION[0003]Surface Acoustic Wave (SAW) resonators have been widely applied to the design of surface acoustic wave filters for use in many different communication systems. FIG. 1 depicts a typical SAW resonator, its symbol, and equivalent circuit. The SAW resonator, which generally comprises a transducer embedded between two reflectors, may be fabricated on a single crystal piezoelectric substrate of lithium niobate or lithium tantalate. The transducer is typically a two terminal d...

AI Technical Summary

Benefits of technology

[0011]Embodiments of the present invention reduce radiation loss and its effects in a leaky SAW resonator. Embodiments of the present invention improve the Q of the resonator to enhance the filter performance. One embodiment may comprise a SAW filter having a piezoelectric substrate and transversely opposing bus bars longitudinally disposed on a surface with a plurality of grating pads transversely extending inwardly along the surface from each of the bus bars, and interdigital transducer electrodes disposed along a longitudinal axis on the surface for defining an acoustic aperture across overlapping interdigital transducer electrodes within a transverse dimension thereof, wherein each of the plurality of interdigital electrodes extends from each of the plurality of grating pads, and wherein a metalization ratio for the plurality of grating pads is substantially different from a metalization ratio for the plurality of interdigital transducer electrodes. The metalization ratio of the bus bar grating pad is defined as a ratio of a width dimension of the grating pad and a corresponding periodic separation distance dimension between adjacent grating pads for each of the plurality of grating pads. Similarly, the metalization ratio of the transducer electrode is defined as a ratio of the transducer electrode width to a corresponding periodic separation distance dimension between adjacent transducer electrodes.

[0012]Increasing the metalization ratio of the grating bus bar pads results in the reduction of the velocity of the bus bar grating pad so that the wave velocity of the grating is slower than the wave velocity of the transducer. The slower velocity of the grating bus bar pads produces a wave guiding effect to maintain the wave's propagation within the active area of the transducer aperture.

Problems solved by technology

It is well known that leaky surface acoustic wave resonators exhibit radiation losses over a range of frequencies.

When configured as a band pass filter, this radiation of leaky waves into the bus bars is manifested as extra insertion loss at certain frequencies, often causing notches or ripples in the pass band.

Depending upon the frequency dependence of the radiation of the leaky waves, the performance degradation may be pronounced near the pass band center or it may occur mostly near one or both pass band edges, thus reducing the usable bandwidth and quality factor, Q, of the filter.

Radiation in leaky surface acoustic wave resonators is difficult to accurately model and is, therefore, quite poorly understood.

However, it is not always practical to construct filters using resonators with large apertures.

This reduction of transducer length results in an increase in spectral losses and resistive losses, both of which adversely affect the Q of the filter.

Further, an increment in the bus bar grating pad actually has an adverse effect on the loss of the filter.

Such techniques are largely trial-and-error, and they usually involve much experimentation with various combinations of length, gap and metalization ratio of the active electrodes.

However, the varying overlap of the active aperture results in an apodization loss, which would degrade the Q of the resonator.

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